This invention relates generally to hydraulic pressure accumulators and, more particularly, to constant pressure hydraulic accumulators for use in aircraft hydraulic control systems.
Hydraulic pressure accumulators are well-known components of various types of hydraulic systems. In simple hydropneumatic tank systems a column of air trapped in the top of a tank is compressed when water is pumped into the tank under pressure, as occurs under automatic control each time tank pressure drops below a pre-set value. No piston is required in the tank. In another type of conventional hydraulic accumulator, a piston and cylinder assemblage is connected to a hydraulic line. Hydraulic fluid occupies a volume open to the hydraulic line on one side of the piston and a gas occupies the volume on the other side of the piston. As the fluid pressure in the hydraulic system fluctuates, the gas is alternately compressed and expanded as the piston moves in response to the changing fluid pressure conditions. The compressed gas thus tends to oppose pressure changes and to provide a degree of pressure regulation.
When a gas-backed piston accumulator is used, during periods of slack hydraulic flow demand, the system pump delivers fluid under pressure into the accumulator and thereby compresses the gas entrapped behind the piston until the accumulator fluid pressure reaches system standard pressure, at which point the pump is stopped. When fluid is drawn by the load, the resulting flow (or drop in line pressure) restarts the pump. However, because of the pump's limited flow capacity, the accumulator supplies most of the temporary flow demand. Because of the compressibility of gas, a conventional accumulator employing a relatively large compressed gas reservoir is capable of augmenting pump flow with limited drop in pressure as it empties its contents. However, with an accumulator of practical size and weight, the pressure drop is more than desired and, because of that which does occur, the hydraulic motors and other devices operated by the system must be designed (usually at higher cost, size and weight) to operate over the full range of pressure drop experienced during maximum load flow.
Aircraft hydraulic systems represent an important application of the invention, for instance in the operation of a hydraulic servoactuator for actuating a control surface, such as an aileron. Most of the time the aileron is not used and its actuator draws little or no flow from the hydraulic fluid source. When the aileron is actuated, typically for a brief period of time, the power requirement of its hydraulic actuator can be very high. Conventional accumulators used in such applications suffer from certain disadvantages and limitations. Most importantly, the delivery pressure of such a conventional accumulator in responding to peak fluid flow demands falls more quickly below the standard operating line pressure of the system than is desirable. For example, if the standard operating pressure of the system is 3,000 pounds per square inch (PSI), a typical operating pressure in aircraft hydraulic systems, the accumulator will normally be fully charged to an air pressure which acts upon the piston of the accumulator to exert a pressure of 3,000 PSI upon the hydraulic fluid. The actual gas pressure, of course, may be higher or lower than 3,000 PSI in the fully charged state, depending upon the ratio of gas to hydraulic pressure areas acting on the accumulator fluid drive piston. Upon the occurrence of a sudden peak flow demand and a corresponding draw of hydraulic fluid from the pump and accumulator, the compressed gas acting on the piston will force hydraulic fluid into the system at a pressure which initially corresponds to the system operating pressure of 3,000 PSI, but which continuously decreases as the gas expands. Since the actuators and other components of the aircraft hydraulic system are designed for operation at or near the standard operating pressure, their performance progressively deteriorates as the line pressure drops and their design must be compromised in efficiency, size, weight and economics for the sake of developing adequate operating power throughout the power stroke.
The primary object and purpose of the present invention is to provide an improved accumulator which can be of limited size and weight yet capable of delivering hydraulic fluid at a pressure that drops during its discharge to a considerably lesser extent than with conventional accumulators of equal volumetric compressed gas capacity.
It is yet another object of this invention to provide an efficient and reliable, trouble-free accumulator system which permits utilization of less expensive, smaller and more lightweight hydraulic components for aircraft hydraulic control systems and similar applications.